Non-patent Document 1 listed below describes focused ion beam (abbreviated as FIB) devices each of which includes a gas field ionization ion source (abbreviated as GFIS) and uses gas ions of hydrogen (H2), helium (He), neon (Ne), or the like. Such gas FIB devices have an advantage of not contaminating samples with Ga unlike gallium (Ga: metal) FIB devices including liquid metal ion sources (abbreviated as LMISs) which are commonly used currently. In Non-patent Document 1, GFISs can form beams finer than Ga-FIB devices because gas ions extracted from the GFISs have narrow energy width and the ion generation source is small.
Non-patent Documents 2 and 3 and Patent Document 1 listed below disclose that provision of a microprotrusion (emitter tip) to the tip end of the emitter of a GFIS or reduction of the number of atoms at the tip end of the emitter to a few atoms provides improvements in characteristics of the ion source, such as an increase in angular current density of the ion source. As an example of fabrication of such a microprotrusion, Patent Documents 2 and 3 disclose fabrication from tungsten (W) of the emitter material by field evaporation. Non-patent Document 3 listed below discloses fabrication of the microprotrusion using a second metal different from the emitter material of a first metal.
Non-patent Document 2 and Patent Documents 2 and 3 listed below disclose scanning charged particle microscopes including GFISs configured to emit ions of He as a light element. From the viewpoint of weight of irradiation particles, a He ion is about 7,000 times as heavy as an electron but is light having a weight of about 1/17  of a Ga ion. Accordingly, sample damage depending on the magnitude of momentum transferred to atoms of the sample from the irradiating He ions is a little larger than that of electrons but is very smaller than that of Ga ions. Moreover, the region where secondary electrons are excited by penetration of the irradiating particles into the sample surface is more localized in the sample surface than that in the case of electron irradiation. Accordingly, it is expected that images by the scanning ion microscope (abbreviated as SIM) are more sensitive to information of the sample surface than images by scanning electron microscopes (abbreviated as SEM). Furthermore, from the viewpoint of microscopes, the effect of diffraction in convergence of an ion beam can be ignored since ions are heavier than electrons. Accordingly, the SIMs are characterized by providing an image with a very large depth of focus.
Non-patent Document 3 listed below states that an ion current can be increased by decreasing the temperature of the emitter tip in the GFIS. Non-patent Document 3 also states that even if the temperature is decreased lower than around the devaporization point (boiling point) of the gas, the ion current is not increased, but on the contrary reduced in some cases.
Patent Document 3 listed below states that the GFIS uses a gas mixture. The component ratio of the added gas is very low, and the purpose of the added gas is not clear. According to the description of the specification thereof, it can be thought that the added gas is expected to contribute to formation or reproduction of the tip end of the emitter tip or contribute to stabilization of the ion source. Moreover, the same document states that the GFIS includes plural independent gas supply means.
Patent Document 4 listed below states that first and second gasses are taken into an emitter region for generation of ion beams of the first and second gasses.
Patent Document 5 listed below describes an ion source including two or more gas introduction lines in order to switch between a gas ion beam type for processing of a sample and a gas ion beam type for observation of the sample.